Ink jet printing is a non-impact method for producing printed images by the deposition of ink droplets in a pixel-by-pixel manner to an image-recording element in response to digital data signals. There are various methods that may be utilized to control the deposition of ink droplets on the image-recording element to yield the desired printed image. In one process, known as drop-on-demand ink jet, individual ink droplets are projected as needed onto the image-recording element to form the desired printed image. Common methods of controlling the projection of ink droplets in drop-on-demand printing include piezoelectric transducers and thermal bubble formation. In another process, known as continuous ink jet, a continuous stream of droplets is charged and deflected in an image-wise manner onto the surface of the image-recording element, while un-imaged droplets are caught and returned to an ink sump. Ink jet printers have found broad applications across markets ranging from desktop document and photographic-quality imaging, to short run printing and industrial labeling.
The ink compositions used in various ink jet printers can be classified as either dye-based or pigment-based. A dye is a colorant, which is dissolved in the carrier medium. A pigment is a colorant that is insoluble in the carrier medium, but is dispersed or suspended in the form of small particles, often stabilized against flocculation and settling by the use of dispersing agents. Commonly used carrier media include water, mixtures of water and water-miscible organic compounds, and high boiling organic solvents such as hydrocarbons, esters, ketones, etc. Most of the ink compositions used in today′s ink jet printers contain water and water-miscible organic compounds.
Materials used in ink jet printing ink compositions must have the correct properties to provide ink compositions that are stable, possess good printing properties, and provide an image with good color, sharpness and image stability. Many colorants are known and used in ink jet printing ink compositions. Many have some or most of these desirable properties but it is very difficult to find a colorant that possesses all of the above properties. Colorants are degraded by ozone and light, and the extent of degradation can differ depending on the components of the ink composition as well as the recording element upon which they are printed. There is a great need to develop ink compositions which have high optical densities, superior lightfastness, and superior ozonefastness when printed on different types of recording elements, in particular, those that are fast drying or porous. It is difficult to find colorants that meet all of these requirements.
Metallic azo complexes are an important class of azo colorants that are commonly used in ink jet ink compositions. Metallic complexes of various azo dyes may be divided into two classes: metallized azo colorants in which the azo group does not participate in the complexation with the metal, i.e., is not a coordination site, and those in which the azo group is one of the coordination sites. The latter complexes have more commercial applications because they exhibit better lightfastness as compared to the former.
Metallic azo complexes can usually have different ratios between ligands and metal ions, e.g. 1:1, 2:1, and 3:1, etc. Also, depending on the number of coordination sites, ligands can be monodentate, bidentate, tridentate etc. Usually, the more coordination sites in a ligand, the better the chemical stability of the complex. For a comprehensive review on metal complex colorants, including metallized azo dyes and pigments see “Metal Complexes as Specialty Dyes and Pigments” by P. Gregory in Comprehensive Coordination Chemistry II-From Biology to Nanotechnology Elsevier Ltd., 2003; Chapter 9.12, pp549-579.
More recent examples of metallized azo dyes being used in inkjet inks are described in U.S. Pat. No. 6,001,161; U.S. Pat. No. 5,980,622; U.S. 2003/0088077 A1; U.S. 2003/0172839 A1; U.S. 2003/0159616 A1; U.S. 2002/0139281 A1; EP 1284200 A2; GB 2372750 A; WO 2004/011560 A2; WO 2004/011561 A1; WO 2004/011562 A1; U.S. Pat. No. 5,725,641; U.S. 2001/0027734 A1; U.S. Pat. No. 6,302,949 B1. In general, these dyes have excellent lightfastness and good color but the ozone fastness on fast drying or porous media is not as good as one would like.
There is still needed an inkjet ink composition with good lightfastness and color t that also has improved ozone stability.